1. Field of the Invention
This invention relates to a television receiver for terrestrial broadcasting effectively used for multichannel television receivers.
2. Description of the Prior Art
Television broadcasting uses the terrestrial or CATV system. With the recent trend that the broadcasting channels have been increasingly multiplied, the CATV system broadcasting has multiplied as 100 channels have been announced. As a result, television receivers are required to be more improved in immunity than would be obtained in the prior art. In such a receiver, particularly, a tuner performs the frequency conversion of an input signal by a local oscillation signal. That is, the heterodyne system has been adopted which selects a desired signal as an intermediate frequency signal. Accordingly, an image frequency signal can be generated. In case that the intermediate frequency is largely apart in value from the input frequency, the image frequency signal also exists in the close vicinity of the input signal, so that it is difficult to be removed by a simple selecting filter.
Under such a circumstance, considerations are made recently on the fact that the upper and lower frequency signals of the local oscillation signal are changed in phase when frequency-converted into the intermediate frequency signal and as a result, such a method that the image frequency signals after frequency conversion are offset has been investigated. One example is disclosed in Japanese Laid-Open Patent Application No. 2-113608, and which comprises two mixers, a biaxial orthogonal local oscillation signal generator and a phase shifter for phase-shifting one of the output signals of the mixers. With such a structure, however, the phase shifter has a frequency limitation on the range of a 90-degree phase shift, making it impossible to follow the frequency change of a converter and resulting in the phase-shift amount not being 90 degrees. Thus, there arises a problem in that the image rejection performance is rapidly degraded.
In addition, another example is disclosed in Japanese Laid-Open Patent Application No. 3-60206, in which two mixers and a biaxial orthogonal local oscillation signal generator are provided and the phases of the output signals of the two mixers are compared to feedback the result to one of phase shifters so as to thereby offset the image frequency signals, and further in which the amplitudes of the output signals of the two mixers are compared to obtain the phase difference and the amplification factors of variable gain amplifiers which are disposed so as to receive the outputs of the mixers are changed in response to the phase difference so as to thereby offset the image frequency signals. With this structure, however, it is very difficult to change closely the amplitudes of the image frequency signals from the output signals of the mixers and as a result, it is required that there be no conversion gain difference between the two mixers in advance. In addition, two phase shifters or two variable gain amplifiers are needed, resulting in redundant circuit arrangement.